1. Field of the Invention
The present invention relates to Integrated Services Digital Networks (ISDN) and more particularly, to a procedure for the activation of tandem digital subscriber lines (DSL) which are separated by carrier systems of the type employing the 3-DSO Time Division Multiplexing (3-DSO TDM) method of ISDN Basic Rate Transport.
2. Description of the Prior Art
ISDN provides a wide-range of voice and non-voice services within the same network, using a limited set of connection types and multi-purpose user-network interface arrangements. ISDN specifications provide for basic access services known as 2B+D. This system provides for two B channels of 64 Kbit/s and one D channel carrying 16 Kbit/s for data transmission. These channels are clear channels fully available to the customer for transmission of information such as voice calls, circuit switched data or packet-switched data. Overhead is transmitted using additional bandwidth in an overhead channel which contains `indicator bits` that specify actions to be taken by various network elements.
The link between an ISDN switch, exchange termination (ET) or a remote digital terminal and the ISDN user is a critical component in the end-to-end digital path. Since a majority of potential users are presently served by a single twisted pair of telephone wires, full-duplex service at rates sufficient to accommodate ISDN basic access must be provided over this single pair. A DSL that provides the required link using a single pair of wires is specified in the May 1, 1990 Draft Addendum to ANSI T1.601-1988, entitled, "Integrated Services Digital Network (ISDN)--Basic Access Interface for Use on Metallic Loops for Application on the Network Side of the NT (Layer 1 Specification)", which document is incorporated herein by reference.
The data transmitted over the DSL requires a 2B1Q line code, which is a four-level pulse amplitude modulation code without redundancy. The line code groups pairs of binary digits into two-bit fields for conversion to quaternary symbols, which are called `quats`. In each pair of bits formed, the first bit is called the `sign bit` and the second is called the `magnitude bit`. Thus, there are available two magnitudes, each at a positive or negative value, for a total of four levels for each quat.
The information flowing across an interface to the DSL is arranged in frames and superframes. Each frame has a 1.5 ms duration and comprises a sync word having nine quats, or 18 bits, 12 2B+D user data channels comprising 108 quats, or 216 bits, and an overhead channel known as the M channel, comprising three quats, or six bits. The overhead channel bits are known as the maintenance bits, or M-bits. The frames are organized into superframes comprising eight frames and having a duration of 12 ms. The first frame in the superframe is identified by inverting the polarity of the sync word in the first frame. As previously mentioned, each frame has six overhead or M-bits; therefore, each superframe is provided with 48 M-bits for control of the system.
The 48 M-bits are identified using two subscripts, such as M.sub.ij, wherein `i` indicates the basic frame number of the superframe, and `j` indicates the M-bit within the basic frame. One such maintenance bit is identified as M.sub.14 and is known as the activation (ACT) bit. This bit plays a key role in establishing the ISDN basic access service capability, in that it indicates the readiness or lack thereof of layer 1 to support higher layer communication over the entire access. ACT bits are used in both directions by DSLs in the startup process, with the transition from 0 to 1 indicating transparency. In the network termination-to-network, upstream, direction, the ACT bit remains set equal to zero until the customer equipment is ready to transmit. When the customer equipment is ready to transmit, this progress indication is indicated by a transition of the upstream ACT bit from 0 to 1. After an ACT=1 is received from the network termination (NT), the ET sets the downstream ACT bit equal to 1 to communicate its readiness and transmits this back to the NT. Once ACT bits equal to 1 are achieved in both directions, the ET and NT are ready for layer 2 communications.
Appendix C of ANSI T1.601-1988 dated May 1, 1990 describes in detail the required activation process for an ISDN connecting an ET to an NT.
Bellcore Technical Reference TR-TSY-000397 Issue Oct. 1, 1988, entitled, "ISDN Basic Access Transport System Requirements", which is incorporated herein by reference, describes the functional and operational aspects needed to define a loop transmission system built around a pair of DSL transceivers which connect one end of a DSL to a line unit (LT) of an ET or an LT-like line unit (LULT), and the other end of the DSL to a line unit of an NT or to an NT-like line unit (LUNT).
When ISDN basic access is to be multiplexed over a higher rate digital transmission facility, the 3-DSO TDM method may be utilized, wherein three DSO channels of the carrier system are utilized for transparent transportation of the B1, B2 and D channels. The DSL overhead is contained in the third DSO channel, along with the ISDN D channel.
In situations where tandem DSLs are used in an access configuration and in particular, where the DSLs are separated by a carrier system, special care must be taken to properly convey ACT bit information status between the ET and the NT, since carrier systems introduce one or more additional facility segments. Tandem DSL systems, separated by an intervening carrier facility, must both be started up before ACT bit confirmations reach the ET and the NT. The necessity of relaying DSL `activation request` indication information, ACT bits, (as defined by ANSI T1.601-1988, Appendix C) across the carrier system was left for further study by Bellcore and ANSI. The resolution of this problem is critical to proper startup of tandem DSL systems and proper ACT bit indications.